A measurement volume is irradiated by means of a light source in the scattered light measurement. If scatter centers, for example dust grains or other particles, are located in the measurement volume, the light is scattered. A light receiver set up at an angle to the irradiated light registers this scattered light which allows conclusions on the kind and quantity of the particles in the measurement volume. The scattered light measurement is used, for example, in environmental metrology or in emission technology.
Over the course of time, the measurement precision can be impaired by contamination or by other effects. To compensate this, a self-test function is known which is actually prescribed with proof-tested emission measurement devices. A possibility of a self-test comprises testing the optical boundary surface of the light receiver with the light source. For this purpose, the light receiver is moved from its measurement position into a test position in which the light is received in a direct line of sight without scattering in the measurement volume. An expected reception intensity in the test position corresponds to the maximum signal so that a representative transmission measurement of the optical boundary surface is made possible. If the reception optics is contaminated, the expected signal is correspondingly attenuated, from which a conclusion can be drawn on a contamination and its degree.
The prior art is familiar with different constructions with which the light receiver can be moved into the beam of the light source. In the simplest case, the light transmitter and the light receiver are pivoted with respect to one another until they are in a direct line of sight. To avoid an overmodulation, an attenuation filter can be arranged in the line of sight. If impairments of the reception optics are to be detected in a spatially resolved manner, a relative movement during the self-test is provided so that the light beam scans the reception optics bit by bit. The light detector can be pivoted on a circular path, for example. However, this has the disadvantage that the light is incident at different angles during the self-test and thus falsifies the result.
A further conventional sliding mechanism arranges the light receiver at the free end of a leaf spring. The leaf spring is bent by a slider for the self-test until the light receiver is in the line of sight of the light transmitter. Small movements of the plate spring are then sufficient to achieve the desired scanning. The light receiver also moves on a circular path with this design. In addition, the plate spring can suffer fatigue from the changing mechanical strains and can break in the worst case.
It is known from EP 1 881 319 B1 to check a reception optics of an apparatus for measuring the light scatter in that the reception optics is guided by a positive cam. This positive cam has two sections, with the reception optics being swept over on the first section by the transmitted light beam on a movement and a test thus being made while the second section provides that the reception optics is moved into the initial test position. This design solves the problem, but it cannot be used for all scattered light measurement devices from a construction aspect.
Overall, the known mechanisms have admittedly proved themselves in some applications, but do not always satisfy the demands on precision, take up too much construction space or are not suitable for all applications from a construction aspect for other reasons.